The attached patch allows one to detach USB-HID devices with usb_del.  
It adds the necessary logic to switch back and forth between PS/2 and 
USB-HID devices.  I've tested with Windows and Linux guests (using the 
evtouch driver).  See the changeset commit message for more details.

I wasn't confident on some of the changes (like where I hooked USB 
device detach) so feedback is greatly appreciated.

Regards,

Anthony Liguori